The following discussion of the background to the invention is intended to facilitate an understanding of the present invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of the application.
One application where signal analysis instruments are used is the measurement of phase noise. There are two general types of phase noise measurement, oscillator phase noise measurement and residual phase noise measurement of a two-port device such as an amplifier.
Two common measurement techniques used to measure the phase noise of oscillators involve the measurement of a single oscillator using a delay line, or the measurement of two phase-locked oscillators.
FIG. 1 shows an example configuration of a single oscillator phase noise measurement. In this configuration, an output signal from an oscillator 1 is input to an LO port of a mixer 2 and to an RF port of the mixer 2. A delay line 3 and a variable phase shifter 4 are provided between the oscillator 1 and the RF port of the mixer 2.
The delay line 3, generally a fixed length of cable or equivalent, de-correlates the signals appearing at the RF and LO ports of the mixer 2. The delay line 3 must have an electrical length that is many wavelengths at the frequency of the oscillator 1 to de-correlate the signals sufficiently for a phase noise measurement. The variable phase shifter 4 allows finer adjustment, generally less than a wavelength, of the relative phase between the signals appearing at the RF and LO ports of the mixer 2. The phase shifter 4 is necessary to ensure the signals appearing at the RF and LO ports of the mixer 2 are in quadrature to ensure the mixer 2 is phase-sensitive.
The output of the mixer 2 represents the phase noise of the oscillator 1, and is input to a signal analysis instrument 5. The signal analysis instrument 5 is used to analyse the spectral density of the output from the mixer 2.
The measurement configuration shown in FIG. 1 has practical limits, imposed by the length of the delay line 3 and signal loss through the delay line 3, on the sensitivity of the phase noise measurements that can be made. One upshot of this limit on the sensitivity of the measurement configuration is that the phase shifter 4 can be of an active design, such as a varactor phase shifter; although active phase shifters generate their own noise it does not impact on the sensitivity of the measurement configuration shown in FIG. 1.
FIG. 2 shows an example configuration of a two-oscillator phase noise measurement. Like reference numerals are used to denote like parts to those shown in FIG. 1. In this configuration, an output signal from a second oscillator 6 is input to the LO port of the mixer 2 and the output signal from the oscillator 1 is input to the RF port of the mixer 2. The output of the mixer 2 represents the sum of the phase noise of the oscillators 1 and 6, and is input to the signal analysis instrument 5. The output of the mixer 2 is also input to a phase-locked loop circuit 7, which is used to phase lock the oscillator 1 to be in quadrature with the second oscillator 6.
The measurement configuration shown in FIG. 2 does not require a delay line, and accordingly can offer superior sensitivity to the configuration in FIG. 1.
FIG. 3 shows an example configuration of a residual phase noise measurement of a two-port device 8. Like reference numerals are used to denote like parts to those shown in FIG. 1. In this configuration, an output signal from the oscillator 1 is input to the LO port of the mixer 2 via the variable phase shifter 4 and to the device 8. An output from the device 8 passes is input to the RF port of the mixer 2.
The output of the mixer 2 represents the residual phase noise of the device 8, and is input to the signal analysis instrument 5. The measurement configuration shown in FIG. 3 can perform high sensitivity measurements, however the sensitivity can be limited by noise and insertion loss from the phase shifter 4 if varactor phase shifters are used. Accordingly, mechanical phase shifters such as trombone phase shifters are used where high sensitivity is required.
In each of the measurement configurations shown in FIGS. 1 to 3, it is necessary to measure the sensitivity with which the mixer 2 converts phase variations in the signals present at its LO and RF inputs to its output signal at its IF port. This sensitivity is commonly called conversion sensitivity or conversion ratio, and is expressed in Volts per radian. In order to produce an accurate spectral density measurement of phase noise, the instrument 6 must take into account the conversion factor when measuring the output from the mixer 2.
The conversion factor is the slope of the voltage output from the mixer 2 at the zero-volt crossing, and has units of volts per radian. A common simplification used to determine the conversion factor assumes that the output from the mixer 2 is sinusoidal. The slope of a perfect sinusoid at its zero-crossing is equal to its peak amplitude. Thus if the output of the mixer 2 is a perfect sinusoid, the conversion ratio can be determined by measuring the amplitude of the sinusoid.
The conversion factor for the single oscillator phase noise measurement configuration shown in FIG. 1 is determined by adjusting the variable phase shifter 4 while observing the change in output voltage from the mixer 2. Where varactor phase shifters are used as the variable phase shifter 4, adjusting the phase shifter is achieved by altering the bias voltage applied to the varactors.
The conversion factor for the two-oscillator phase noise measurement configuration shown in FIG. 2 is generally determined from a sinusoidal beat signal observed at the output of the mixer 2, which is obtained by disconnecting the phase-locked loop circuit 7 and adjusting the frequencies of the oscillators 1 and 6 until a suitable beat signal is obtained.
The conversion factor for the residual phase noise measurement configuration shown in FIG. 3 is determined by adjusting the phase shifter 4 while observing the change in output voltage from the mixer 2.
Some instruments calculate the conversion factor for single-oscillator or two-oscillator phase noise measurements. However determining the conversion factor for a residual noise measurement is still a manual process that is prone to error and laborious.
Phase noise, and its measurement, is described in more detail in NIST Technical Note 1337.